Oscillator drift compensation



Nov. 24, 1942. c. N. KIMBALL OSCILLATOR DRI ET COMPENSATION DEVICE Original Filed Sept. '7, 1940 INVENTOR ,Zwflmall ATTORN EY O kar E i l Patented Nov. 24, 1942 r OSCILLATOR DRIFT COMPENSATION DEVICE Charles N. Kimball,

Kansas City, Mo., assignor to Radio Corporation of America, a corporation of Delaware Original application 355,714, now Patent 21, 1942. Divided an September 7, 1940, Serial No. No. 2,280,527, dated April d this application September 16, 1941, Serial No. 410,968 1 '7 Claims.

My present invention relates to devices for reducing oscillator frequency drift due to thermal effects, and more particularly to electronic de- ,vices adapted to minimize oscillator drift at ultra-high radio frequencies. This application is a division of my application Serial No. 355,714, filed September 7, 1940, patented April 21, 1942, as U. S. Patent No. 2,280,527.

One of the main objects of my present invention is to provide a frequency drift compensation device for the tank circuit of an oscillator, especially one operating at ultra-high frequencies, wherein the compensation device comprises an electronic device constructed and arranged to simulate across the tank circuit a reactive effect capable of compensating for the reactive effect causing the frequency drift.

Another important object of this invention is to improue frequency drift compensation devices,- and the latter essentially comprising a tube having a mutual conductance of a predetermined polarity sign, a phase shift network being associated with the input and output electrodes of the tube to produce between the output electrodes a capacity effect whose sign of temperature coefiicient is the same as, or reverse of, the sign of temperature coefiicient of a capacity included in said phase shift network, but whose magnitude of temperature coefiicient is altered by the capacitive multiplying action of the control tube.

Another object of the invention is to provide a capacity having a negative temperature coeificient of arbitrary value by utilizing a tube having a negative mutual conductance to provide between its output electrodes the said negative coefficient, a phase shift network, employing a capacity of positive temperature coefficient, being arranged in the tube circuits to provide a phase quadrature relation between the input and output electrode potentials of the tube.

Still another object of my invention is to provide a positive mutual conductance tube with a phase shift network embodying a positive temperature coefiicient condenser, and there being produced between the tube reactive efiect having a positive temperature coefiicient.

Yet other objects of my invention are to improve generally the efiiciency and reliability of frequency drift compensation devices, and more especially to provide frequency drift reducing networks, adapted for ultra-high frequency oscillation circuits, which are economically manufactured and assembled,

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in local oscillator network be provided with *of a superheterodyne receiver, the locally-prooutput electrodes a minutes of warm-up connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Figure 1 shows an oscillator circuit embodying,

the invention,

Figure 2 illustrates a modification, Figure 3 shows another modification.

Referring now to the accompanying drawing, wherein like reference characters in the difierent figures designate similar circuit elements, there is shown a tube l which may be of any wellknown type. The tube is included in the tunable of a radio receiver of the superheterodyne type adapted to receive ultrahigh carrier frequencies. For example, such carrier frequencies may be included in the frequency modulation (FM) band of approximately 40 to 50 megacycies (me). Of course, the present invention is not restricted to use in a radio receiver. Generally speaking it is directed to compensation of frequency drift in any oscillation circuit. Hence, by way of illustration, let tube a cathode 2, a control grid 3 and a plate 4, and assume that coil 5 and variable condenser 63 provide the usual tunable tank circuit connected between the grid and cathode. The usual grid leak resistor I, shunted by the capacity 8, is inserted in the grid side of the tank circuit. The plate is connected to the customary positive voltage source through a feedback coil 9 coupled reactively to tank coil 5. Where the circuit is used as the local oscillator duced oscillations may be taken off from the grid and applied to the first detector, or mixer, tu e.

Now it is found that during the first 30 to time frequency driftoccurs in the tank circuit 56 due to changes in the values of the frequency-determining reactive eleelements with temperature may be denoted by changes in the capacity of variable condenser 6. Usually such drift has been corrected by paralleling the tuning capacitance with a small condenser known to have an appropriate value of negative temperature coefficient, it being assumed, of course, that the capacitance of the circuit has a positive temperature coeflicient.

According to the present invention a highly flexible and efficient electronic device is used in place of a physical shunt condenser. A tube is utilized in such a manner that its plate to cathode impedance is essentially a capacitative load across the tank circuit, or a portion thereof. In Fig. l the reactance tube is designed by the numeral l0, and it may be provided with a cathode l I, an input grid l2, an auxiliary grid l3 and a plate M. The plate is fed with a proper positive voltage through a radio frequency choke, while a phase shift network comprising condenser 15 and a series resistor 16 are connected between the plate l4- and the grounded cathode. The negative biasing source I1 is connected between the lower end of resistor I6 and ground.

The grid I2 is connected to the junction of condenser l and resistor l6. Hence, the grid l2 has a fixed negative bias relative to its cathode. The plate l4 may be coupled to any desired point along coil 5 by the condenser l8, and an adjustable tap l may be used to provide the simulated capacitance 20 across any desired portion of the tank coil 5. The condenser l5 may have any arbitrary temperature coefficient in this form of the invention The temperature coefficient (K) may be positive or negative, but not zero. By way of illustration, let it be assumed that the tuning capacitance of the tank circuit has a positive temperature coefficient. It is desirable, then, that the simulated capacitance 26 have a negative temperature coefiicient so as to compensate for the capacity change with temperature in the tank circuit. This is readily accomplished by choosing a condenser l5 whose K is positive, and utilizing for tube Ii! one having a negative mutual conductance (gm). The latter is simply secured by applying a substantially higher positive voltage to screen l3 than to plate l4. Those skilled in the art are fully aware of the many ways of producing a gm tube.

It can be shown that the net change in total capacitance across the tank circuit 5--B due to temperature is:

of resistor IS. The expression (1) can be made zero by making:

C1K1 ..R=- g C2K2 Hence, if K2 is positive (value of C2 increasing with temperature) gm must be negative, and

vice versa.

The advantage of the present arrangement resides in the control which is afforded over the effective value of K2. This can be changed merely by adjustment of the gm of tube l0. Thus, effectively,

01K, czgmR If no control tube l0 were used the value of K in the shunt correction condenser would have to be equal in magnitude, and opposite in sign, to the value of Kin the drifting tuning capacie tance. This is of importance to the set designer since it is often difficult to secure physical condensers of just the desired magnitude and sign of temperature coefficient. The present invention enables the designer to choose at random a condenser l5 of any small value, and of any sign for K, and by proper adjustment of tube It! is able to magnify or diminish the value of E5 to the desired value, and even properly adjust the sign of K.

The invention is not limited to use of a negative gm tube at It], since the control tube may be a positive gm tube. In general, the sign of the resultant capacity temperature K as seen at the control tube plate is determined by the quotient of gm and the phase shifter condenser K. Hence, if either gm or K15 is negative, the K20 is negative. If both K15 and gm are positive or negative at the same time, K20 is positive. Thus, Fig. 2 shows tube Ill of the high plate impedance type, as a pentode, while the phase shifting network consists of condenser l5 whose K is positive and series resistor IS. The input grid I2 is connected to the junction of IS and 16. The reactive effect 20 in this case is a capacitance having a positive K. The cathode lead may include an adjustable resistor 30, properly by-passed, to permit initial adjustment of the negative bias of grid 12 thereby to control the gain of tube Ill. As in the case of Fig. 1, the resistor I6 should have a sufficiently small magnitude to cause substantially a degrees phase shift between the plate potential and input grid potential. Under these conditions the plate to cathode impedance of tube It! (01 I0) presents a capacitance effect across the tank circuit. Those skilled in the art are fully aware of this phenomenon. It can readily be shown that the plate to cathode impedance has a reactance equal to that which would be caused by a condenser whose magnitude and sign depends upon the magnitude and sign of the tube mutual conductance and on the value of the shifting network resistor. As the ambient temperature increases the magnitude of the tuning capacitance varies. Automatically the magnitude of capacity l5 varies and by virtue of the multiplying effect of tube It) is able to produce a change in simulated capacitance 20 to an extent, and in such a direction, that the tuning capacitance change is accurately compensated for. This invention can readily be utilized for the ultra-high radio frequencies of the FM band. With the small variations in tuning capacitance encountered in tuning from one end of the FM band to the other the arrangement is as inherently capable of reducing oscillator drift as would be the more conventional direct application of a shunt condenser of negative or positive K to the oscillator tank.

In Fig.3 there is shown another use of the invention. In this embodiment the phase shifter condenser 15 has a substantially zero K. The cathode of tube 10, which may be a pentode, is grounded. .The grid I2 is connected to a negative potential point on voltage divider resistance 50 through a path comprising resistor l6, lead 5| and tap 52. The condenser 53 by-passes the resistor 16 to ground. Resistor 56 may include a bi-metallic or other temperature-dependent element so that the control tube grid bias is caused to change in a manner such that the frequency drift is maintained at a low value. The resistor 50 is connected across a direct current voltage supply source of negative polarity, say

about 20 volts. Tap 52 may be applied to a point which is about 3 volts relative to ground.

In the arrangement of Fig. 3 the gm of tube Hl' varies with temperature, while maintaining C15 constant. Thus, K15 is zero, and the control action'would be effected by changes in gm. The latter effect is produced by causing the tube bias to vary with temperature. This may be done by employing a special construction for divider 50. The divider resistor 50 is divided into two sections R1 and R2. One section has constant resistance not varying with temperature. The other section has a resistance varying with temperature. Since in most pentodes gm varies approximately linearly with bias it follows that the effective K of the tube is the K of the resistor which varies with temperature in the divider. Thus, if it is desiredto have gm decrease as the temperature rises, which would cause the net output capacitance of the control tube to fall as the temperature rises, that is the effect of a negative K, one can employ elements in the reactance tube grid circuit which have substantially zero change with temperature. There are several possibilities, all of which depend upon the fact that R2 is substantially greater than R1. This permits the assumption that I=E/R2. The bias circuit may then be considered as a "constant current circuit. Either R1 or R2 may be variable with temperature, although both may be variable if their temperature coefficients differ sulficiently to provide a reasonable relative resistance Variation with temperature.

R1 and/or R2 may be obtained in either positive or negative K commercially. For example, wire wound resistors (copper, nickel, etc.) have positive K. Carbon has a negative K. Based on an assumption of R2 being greater than R1 the following combinations are possible:

Apparent R; R, G... of tube 10 capacitance of I reactance tube Constant with Rises wi t h Rises as temp. R i s e s a s temp. temp. (+K). rises (+K). temp. rises Rises with Constantwith Falls with Falls with temp. (+K). temp. temp. (K). temp. (K). Falls as temp. Constant with Rises astemp. R i s e s a s rises (K). temp. (+K). temp. rises +K). Constant with Falls astemp. Falls with Falls with temp. rises (K). temp. (K). temp. (K).

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a resonant circuit adapted to vary in frequency by virtue of temperature effect on the circuit reactance, an electron dis-charge tube having at least a cathode and two cold electrodes, means applying alternating voltage across said circuit between said cathode and one of said cold electrodes, a phase shifter connected with said one cold electrode for developing from said voltage a second voltage in phase quadrature therewith, means applying the second voltage between the second cold electrode and cathode whereby a reactive eflect is developed between said one electrode and cathode, said phase shifter including a condenser having a zero temperature coefficient, means for varying the mutual conductance of said tubeto adjust said reactive effect sufiiciently to compensate for said frequency variation, said varying means being temperature-dependent.

2. In combination with a resonant circuit adapted to vary in frequency by virtue of tem-' perature effect on the circuit reactance, an electron discharge tube having at least a cathode and two cold electrodes, means applying alternating voltage across said circuit between said cathode and one of said cold electrodes, a phase shifter connected with said one cold electrode for developing from said voltage a second voltage in phase quadrature therewith, means applying the second voltage between the second cold electrode and cathode thereby to develop a predetermined reactive effect between said one electrode and cathode, said phase shifter including a. reactance having a zero temperature coefficient, means for varying the mutual conductance of said tube to adjust said reactive effect for compensating for said frequency variation, said varying means being temperature-dependent and including at least one temperature-dependent resistor.

3. A frequency drift compensation device comprising a tube having a cathode and at least two cold electrodes, means supplying voltage of the frequency to be compensated between the cathode and one cold electrode, temperature-constant means for deriving from said voltage a second voltage in phase quadrature therewith, means applying the second voltage to the second cold electrode whereby a reactive effect is produced between said cathode and one cold electrode, and temperature-dependent means for adjusting the gain of said tube thereby to adjust said reactive effect in a manner to compensate for frequency deviation of said first voltage from a predetermined frequency.

4. A frequency drift compensation device comprising a tube having a cathode and at least two cold electrodes, means supplying voltage of the frequency to be compensated between the cathode and one cold electrode, temperature-constant means for deriving from said voltage a second voltage in phase quadrature therewith, means applying the second voltage to the second cold electrode whereby a reactive effect is produced between said cathode and one cold electrode, and temperature-dependentmeans for adjusting the gain of said tube thereby to adjust said reactive eifect, said adjusting means including a potentiometer having at least a section thereof variable with temperature.

5. A frequency drift compensation device comprising a tube having a cathode and at least two cold electrodes, means supplying voltage of the frequency to be compensated between the cathode and one cold electrode, temperature-constant means for deriving from said voltage a second voltage in phase quadrature therewith. means applying the second voltage to the second cold electrode whereby a reactive eifect is produced between said cathode and one cold electrode, and temperature-dependent means for adjusting the gain of said tube thereby to adjust said reactive effect, said adjusting means including a potentiometer having at least a section thereof variable with temperature, and means for variably connecting said second cold electrode to said potentiometer.

6. A temperature-dependent variable reactance device comprising an electron discharge. tube having at least a cathode, a positive cold electrode and a control electrode, means or applyingalternating voltage between the positive electrode and cathode, a phase shifter, including a reactance element having substantially zero temperature coeflicient, connected between said positive electrode and cathode, means deriving from said shifter a second alternating voltage in substantially phase quadrature with the first voltage, means applying said quadrature voltage to said control electrode thereby to provide between said cold electrode and cathode a reactive effect, and temperature-responsive means for varying the potential difference between said control electrode and cathode thereby to vary the magnitude of said reactive effect.

'7. A temperature-dependent variable reactance device comprising an electron discharge tube hav- Ill ing at least a cathode, a positive cold electrode and a control electrode, means for applying a1- ternating voltage between the positive electrode and cathode, a phase shifter, including a reactance element having substantially zero temperature coefficient, connected between said positive electrode and cathode, means deriving from said shifter a second alternating voltage in substantially phase quadrature with the first voltage, means applying said quadrature voltage to said control electrode thereby to provide between said cold electrode and cathode a reactive efiect, and temperature-responsive means for varying the potential difference between said control electrode and cathode thereby to vary the magnitude of said reactive effect, said last means comprising a potentiometer having at least a section thereof constructed to be temperature-dependent.

CHARLES N. KIMBALL. 

